I-III-VI Compound Semiconductors for Water Splitting
LaboratoryNational Renewable Energy Laboratory (NREL)
Capability ExpertKai Zhu, Christopher Muzzillo
Process and Manufacturing Scale-Up
Node Readiness Category1: Photoelectrochemical (PEC)
The CIGS team at NREL has the ability to grow high-quality I-III-VI compound semiconductors for photovoltaic as well as PEC applications. The typical I-III-VI chalcopyrite material is Cu(In,Ga)Se2 (or CIGS). The NREL CIGS team has demonstrated ~20% CIGS photovoltaic device (with ~35 mA/cm2 photocurrent density) based on the standard configuration (glass/Mo/CIGS/CdS/ZnO/ITO) . This performance was once the world record efficiency and is still among the higest obtained so far. Although the bandgap of CIGS for the typical PV application is about 1.1 eV, it can be varied from ~1.0 eV (CuInSe2) to ~1.6 eV (CuGaSe2) to ~2.4 eV (CuGaS2). The NREL CIGS team can fabricate a variety of I-III-VI chalcopyrity semiconductors, such as CuInSe2, CuInS2, CuGaSe2, CuGaS2, CuGa3Se5, alloys of Cu(In,Ga)(S,Se)2 with precise composition control. 2D materials such as GaSe, InSe, (Ga,In)Se, and MoSe can also be fabricated. With a bandgap of ~1.5 eV, the NREL CIGS team has demonstrated an open circuit voltage of ~830 mV . The wide selection of material compositions and continuous bandgap tuning coupled with high photocurrent and voltage generation are very attractive for PEC applications especially multijunction PEC electrodes or hybrid photo-electrodes for H2 production. We have recently demonstrated that defect chalcopyrite (CuGa3Se5) with a bandgap of ~1.8 eV, is promising for PEC cells. The NREL CIGS team has developed the state-of-the-art ultrahigh vacuum (UHV) CIGS cluster tool located in Process Development and Integration Laboratory (PDIL) at NREL. This UHV deposition system is capable of fabricating sample sizes 6"x6" in a variety of substrates (glass, metal foils, high temperature plastics, others). The substrate heater can provide up to 1000oC and the UHV environment allows for accurate incorporation of extrinsic doping levels to a given material. This UHV CIGS tool is part of a cluster tool where other relevant thin-film materials are fabricated (Mo, Cr, other metals by DC sputtering; ZnO, ITO and other ceramic target materials using RF sputtering). A pulsed DC power supply allows for reactive sputtering of metals and/or semiconductors (example Zn(O,S)). The cluster tool is capable of the full fabrication of PV and PEC cells (and prototype mini modules or large area PEC cells) that utilize compatible materials. In addition to the growth system, we also have capabilities to do CdS or Zn(O,S) by chemical bath deposition methods. Finally, other standard characterization techniques (e.g., XRD, Raman, XRF, SEM, AFM, Uv-vis, FTIR, PL) are accessible to us on the routine basis.
Uniform deposition on 6"×6" substrates.
This is the only capability within the national lab system with demonstrated capability of producing record-efficiency CIGS based photovoltaic devices . Most companies and university teams are using ideas and intellectual properties developed by the NREL CIGS Group, including the breakthrough of the NREL "three-stage process" . Our UHV CIGS cluster system is uniquely designed to have the capability to deposit all the necessary layers in a CIGS thin-film PV and PEC cells, including prototype mini modules or large area PEC devices.
We can produce multiple samples/run on substrates of up to 6"×6" size. These large-size substrates can later be tailered to multiple (smaller) sized samples to accommodate various specific project needs. Some of the capabilities were initially designed with open mind to enable easy testing/developing of individual layers with industrial partners or university collaborators. The CIGS cluster tool has been an integral part in the technological work done under WFO, CRADAs and TSA with private sector institutions as well as universities via FPACE and other agreements. Worthnoting is that one ongoing project is to develop wide-bandgap chalcopyrite PEC electrodes for H2 production with a university partner. Standard samples can be provided to EMN partners. We can also work with collaborators to have unique designs to meet the specific project needs/goals.
This described capability can allow users to work with NREL experts to test or develop specific layers while keeping other optimized layers of the device/electrode stack unchanged. This would significantly reduce the developenment cycle. It can also be used to generate standard samples with high reproducibility and realibility in order to help EMN collaborators/partners to develop the specific electrode structures at their own research locations. All these activities will be in direct support of the HydroGEN goals.
Photograph of the state-of-the-art ultrahigh vacuum (UHV) CIGS cluster tool at NREL. This cluster tool offers powerful capabilities with integrated chambers for depositing, processing, and characterizations. Techniques include evaporation; RF/DC/pulsed DC sputtering, and wet-chemical-bath deposition. All but the wet-chemical deposition chamber are maintained at ultra-high vacuum (<5x10-10 torr) to minimize contamination defects.
1. Repins et al, "19.9%-efficient ZnO/CdS/CuInGaSe2 Solar Cell with 81.2% Fill Factor," Prog. Photovolt: Res. Appl. 16, 235-239 (2008). 2. Contreras et al, "Wide bandgap Cu(In,Ga)Se2 solar cells with improved energy conversion efficiency," Prog. Photovolt: Res. Appl. 20, 843–850 (2012). 3. http://www.nrel.gov/ncpv/images/efficiency_chart.jpg. 4. Contreras et al, "High Efficiency Cu(In,Ga)Se2-based Solar Cells: Processing of Novel Absorber Structures," 1st WCPEC (24th PVSC IEEE 94), December 5-9 1994, Hawaii.